1. Field of the Invention
This invention generally pertains to the field of fire alarms and detectors and more particularly concerns a method and apparatus for conveniently testing the operation of heat detector alarm installations.
2. Background of the Invention
Early warning of fire in residential and commercial buildings has been proven to save numerous lives every year and has become a matter of national concern. For this purpose several different types of fire alarm systems are in use, designed to meet the requirements of various kinds of installations. Residential installations typically rely upon smoke detectors, which respond to the presence of air borne smoke particles generated in the early stages of combustion. However, smoke detectors can be unreliable in some commercial and industrial environments due to the presence of other airborne materials, vapors and dusts produced in the normal course of commercial and industrial activity and which can falsely activate smoke detectors. Many commercial and industrial installations therefore depend in part upon heat detectors which are activated by certain changes in temperature indicative of a possible fire.
Most modern heat detectors incorporate either the rate of rise principle of operation or are of the rate compensated type. Each such type of detector is capable of sensing not simply the existence of an elevated temperature, but rather the rate of rise of the temperature of the air surrounding the detector so long as this rate exceeds preset limits. The temperature of air near a ceiling tends to rise rapidly in the event of a fire, and heat detectors incorporating the rate of rise or rate compensation feature are designed to respond to such rapid rise in temperature in order to discriminate against more gradual temperature increases unrelated to conflagrations. Rate compensated heat detectors, on the other hand, are a combination of fixed temperature and rate anticipation i.e., they activate an alarm simply upon reaching a given temperature during slow heat rise. During rapid heat rise, however, they are designed to account for the temperature lag between the detector temperature and air temperature. The temperature of the heat detector unit always lags behind the rising temperature of the surrounding air. This is because it takes a certain amount of time for heat transfer to occur from the ambient air to the heat sensor unit. The extent of this lag depends on how quickly the air temperature is rising, the lag being greater for a faster temperature rise of the air. Rate compensated heat detectors are constructed to compensate for this temperature lag, so as to trigger an alarm at a lower detector temperature if the temperature of the detector is rising rapidly, and trigger the alarm at a higher detector temperature if the rate of rise is slower.
Rate compensation detectors respond when the temperature of the air surrounding the device reaches a predetermined level, if the temperature rise is of a rate less than 5 degrees F./minute, and responds quickly thus minimizing temperature lag when the air temperature rise exceeds 5 degrees F./minute. A rate of rise detector, by contrast, responds when the detector temperature rises at a rate greater than 15 degrees F./minute but does not operate if the temperature rise is slower than 15 degrees F./minute. Most rate of rise heat detectors are combined with a fixed temperature feature. The fixed temperature portion of the combined rate of rise/fixed temperature heat detectors is sometimes activated by a fusible link made of a eutectic material, which can be a metallic alloy characterized by a low melting point. The eutectic alloy is selected to melt at the desired fixed temperature, and may be installed in such a way that an electrical circuit is closed when the fusible element melts. For example, a spring element can be held in a stretched condition so that upon melting of the eutectic element, the spring is released into contact with a second element to make an electrical connection. Eutectic alloy sensors are one shot devices, and must be replaced if once activated. Other models use a bi-metal arrangement which changes shape causing a contact closure at the desired temperature. Such detectors are self-restoring and so are reusable.
A more recent evolution in heat detection is the electronic heat detector. This detector utilizes a thermistor as a sensing element. As a thermistor is an electrical resistor made of a semiconductor whose resistance varies sharply in a known manner relative to changes in temperature it can be used in a variety of heat sensing applications.
Thermistor based heat detectors are electronic as compared to other heat detectors in the fire alarm industry as they are electro-mechanical. Since thermistor based heat detectors are electronic their function is controlled by the design of the electronics which drive and monitor them.
Some thermistor based heat detectors are simply fixed temperature in that when the thermistor reaches a set temperature its known resistance at this temperature creates the electrical change in the circuit which has been designated as the alarm threshold and so the control panel to which the detector is connected, and from which the detector receives its electrical power, signals an alarm.
Variations in the electronics which monitor the resistive change in this thermistor permit some detectors to be used in analog systems, i.e. an analog system continuously accepts the change, or lack of change, in the resistance of the thermistor and interprets this information as either normal or off-normal and reacts by signaling an alarm if the off-normal reading meets a designated threshold.
Thermistor based heat detectors can as well be rate-of-rise or rate-of-rise/fixed temperature simply by the electronics used in the circuit which powers and interprets the return signals from this type heat detector.
Because the thermistor is mechanically somewhat fragile, the heat and even combination heat/smoke detectors which incorporate a thermistor provide an open, lattice like shield over the thermistor for protection from physical damage. This shield isolates the thermistor from coming into contact with a solid heat source and further causes an insulative barrier of air some one eighth to one quarter of an inch between the thermistor and a solid heat source.
The heat source used in U.S. Pat. No. 5,611,620, issued to Wantz, on Mar. 18, 1997, did not provide enough heat to overcome the isolated position of the thermistor and did not cause the electronic, thermistor based heat detectors to be tested.
Because of the higher heat of the exothermic reaction of the composition formulated wafer put forward in this application, the thermistor based detectors can now be tested as well as the electro-mechanical types which were tested by the heat pad in U.S. Pat. No. 5,611,620. Further, the higher heat of the formulated wafer also extends the range of the testable heat detectors into the Intermediate range.
The various types of heat detectors are each available in several temperature ratings, designed to respond at different temperature ranges. The temperature classifications include the Low temperature range from 100 to 134 degrees Fahrenheit, the ordinary temperature range from 135 to 174 degrees Fahrenheit, the Intermediate range from 175 to 249 degrees, and several still higher temperature ranges. The great majority of heat detectors currently in use, however, fall within the Ordinary temperature range, i.e. they activate at about 135 degrees Fahrenheit.
Each heat detector has a radius of effective coverage. This radius varies from one heat detector model to another, and typically is between 25 feet and 50 feet. A typical installation requires a number of heat detectors installed in a grid pattern on the ceiling of the structure to be protected. The spacing between the detectors is determined by the effective coverage capability of each unit. A large commercial or industrial space, such as a warehouse, may have a considerable number of heat detectors. Furthermore, such spaces commonly have high ceilings, which places the heat detectors out of easy reach.
Prior to the invention disclosed in U.S. Pat. No. 5,611,620 issued to Wantz on Mar. 18, 1997, only makeshift methods existed for the operational testing of heat detectors, if such testing was done at all. Commonly employed heat sources included the use of hair dryers, heat guns and heat lamps. A ladder had to be placed under each heat detector and the heat source hand carried up the ladder to test the detector. Long extension cords were typically required by this approach. Such methods were cumbersome, time consuming and ineffective, with the result that too often heat detectors went untested over extended periods, in spite of annual testing requirements by industrial and commercial codes.
The U.S. Pat. No. 5,611,620 patent disclosed a method for testing heat detectors using heat packs containing a powdered iron composition formulated to react exothermically upon exposure to ambient air. Such heat packs are commercially available and are sold for use as personal body warmers in cold environments, and provide a convenient, inexpensive and safely disposable source of flameless heat for activating heat detectors. An extension handle equipped with a holder for the heat pack permits an operator standing on the ground or floor to reach heat detectors mounted high on a wall or ceiling without having to climb on ladders. In combination, the air activated exothermic composition and the extension device provide a considerable improvement over the then existing state of the art.
Nonetheless, the U.S. Pat. No. 5,611,620 patent recognizes a shortcoming inherent in the use of disposable heat packs based on air activated exothermic compositions, in that such heat packs generally develop sustained peak temperatures in the range of about 135 to 165 degrees Fahrenheit. This temperature range is quite suitable for triggering and testing heat detectors in the Low and Ordinary temperature classifications. It is, however, insufficient for testing electronic heat detectors and the regular heat detectors in the Intermediate and higher ranges, which trigger at temperatures of 175 degrees Fahrenheit and above. No satisfactory method for testing these electronic or higher temperature rated heat detectors is presently available.
A continuing need therefore exists for a safe, efficient and reliable method for testing heat detectors based on electronics or those having a temperature rating equal to or greater than about 175 degrees Fahrenheit.